As is known, organic optoelectronic devices such as OLEDs, devices comprising photovoltaic cells, and those comprising organic TFTs, need to be encapsulated to protect their sensitive components from gaseous species in the atmosphere (mainly oxygen and water vapor). This is because, if suitable protection is not in place, there is a risk that the device will subsequently degrade, which degradation manifests itself mainly through the appearance of black, non-emissive spots, in the case of OLEDs, which are in fact a result of water vapor penetrating into the diode, thereby degrading the cathode (or anode)/organic film interface.
This encapsulation may typically be produced using a glass cap bonded onto the organic device using a specific adhesive, especially one having low water permeability. In general, a solid moisture getter is added between the substrate and the cap to prolong the lifetime of the device. Encapsulation with a cap is well suited to rigid devices, but it is not well suited to devices comprising flexible supports (e.g. flexible displays). This encapsulation technique is also unfeasible in the case where space is lacking on the circuit of the substrate, for example in a CMOS (complementary metal-oxide semiconductor) microdisplay, and it is to be avoided if it is desired to minimize the weight of the device, in particular in the case of large emission areas.
In all these cases where encapsulation with a cap is not suitable, what is called “monolithic” encapsulation is generally adopted, i.e. encapsulation using thin films having good oxygen-barrier and water-vapor-barrier qualities, in particular. The materials most commonly used for this application are in general dielectric oxides and/or nitrides of formula SiOx, SiNx, SiOxNy and AlxOy usually deposited using chemical vapor deposition (CVD), optionally plasma-enhanced (PECVD), or atomic layer deposition (ALD), which techniques are preferred to physical vapor deposition (PVD) which, like sputtering, are most of the time too aggressive with respect to organic semiconductors or else lead to the formation of films having properties that are unsatisfactory for barrier applications due to the many pinhole-type defects found in these evaporated films. PECVD and ALD techniques have the advantage of being highly conformal (i.e. they provide excellent step coverage) for the films deposited, which have far fewer defects than films obtained using PVD techniques.
Single inorganic layer encapsulation structures currently obtained using these PECVD or ALD techniques nevertheless have atmospheric gas impermeability properties that are unsatisfactory due to the defects that remain in these layers whatever is done. For example, if it is decided to improve the water vapor impermeability of a commercially available PET film having a permeability of about 1 g/m2/day by depositing on the surface thereof, using low-temperature PECVD, an inorganic film of silicon nitride (the case of food packaging), this impermeability is at best improved by a factor of 100 and the PET/SiNx multilayer will then have, in the best case scenario, a water vapor permeability of about 10−2 g/m2/day.
In order to further reduce this water vapor permeability attempts have recently been made to produce organic/inorganic/organic/inorganic/etc. multilayers, such as those sold under the brand name Barix®, wherein the polymer underlayer of each organic/inorganic dyad is deposited by vacuum evaporation, allowing the defects of one inorganic layer to be “decorrelated” from those of another in order to make the path traveled by the water vapor more circuitous and thus impede the diffusion of the water vapor through the encapsulation structure. In this way, it is possible at the present time to reduce the water vapor permeability of such a structure to values of about 10−6 g/m2/day, thereby procuring sufficient lifetime that commercialization of OLED display devices may be envisioned.
Another large family of multilayer encapsulation structures is marketed by Philips under the brand name “NONON” and consists of a multilayer comprising an alternation of nitride and oxide layers, for example SiNx/SiOx/SiNx/SiOx/etc.
Mention may also be made, for OLED device encapsulation, of the multilayer structure described in document US-A-2007/0184292, comprising an internal organic/inorganic multilayer surmounted by an external multilayer comprising two polymer layers which are joined together by an adhesive layer whereof the two interfaces with these polymer films are formed from two heat-cured films.
A major drawback of multilayer structures comprising alternating organic and inorganic layers, such as Barix® structures, lies in the relatively large thickness (in general greater than 500 nm) of each polymer layer sandwiched between two inorganic layers, thereby leading to a large total thickness for the structure formed from n organic/inorganic dyads, in order to obtain the desired oxygen and water vapor impermeability properties. Thus, for four of these dyads, it is easily possible to exceed a thickness of 2 μm, which is unfeasible for certain devices, such as microdisplays on CMOS substrates, which require very thin encapsulations. This is because, the quality of these microdisplays depends on the aperture ratio between the area of each light-emitting pixel and that of the corresponding color filter, which is usually placed on the internal side of the transparent protective cap bonded to the microdisplay, which ratio is directly related to the distance separating the color filters from the pixels and therefore to the thickness of the encapsulation structure and/or the adhesive surmounting the microdisplay.